We have a 2004 147 8v JTD and a 2007 GT JTD. Not driven a 16V JTD 147 but comparing the two diesel cars we have suggests that both engines have advantages. The 8V is not as fast but low rev torque is better. Going from the GT to the 147 is interesting. For short journeys it actually feels a bit more responsive unless you a deliberately in a hurry. If you want to really nail it then the 16V GT is considerably faster.
Economy is similar. We get around 44-45 day to day with the 147 and 42-43 with the GT. Both will manage 50+ mpg when cruising on the motorway - depending on the average speed of course...
I would recommend driving one of each to see which suits your driving style.
"When you sit in a Focus RS or a Golf R32 itís like sitting in a commercial for Lynx aftershave. When you sit in an 147 GTA itís like sitting in a Venetianís hand-made suitcase." J.Clarkson 2003.
Current fleet: 156 Sportwagon jtd 16V (facelift) 2.4 Spider, Leon Cupra SC
Projects: Spider S4, Matra Murena 2.2